5.3.1 Closing items

The most basic logging is to keep track of when a certain TODO
item was finished. This is achieved with1

(setq org-log-done 'time)

Then each time you turn an entry from a TODO (not-done) state into any of the
DONE states, a line ‘CLOSED: [timestamp]’ will be inserted just after
the headline. If you turn the entry back into a TODO item through further
state cycling, that line will be removed again. If you turn the entry back
to a non-TODO state (by pressing <C-c C-t SPC> for example), that line
will also be removed, unless you set org-closed-keep-when-no-todo to
non-nil. If you want to record a note along with the timestamp,
use2

(setq org-log-done 'note)

You will then be prompted for a note, and that note will be stored below
the entry with a ‘Closing Note’ heading.

In the timeline (see Timeline) and in the agenda
(see Weekly/daily agenda), you can then use the l key to
display the TODO items with a ‘CLOSED’ timestamp on each day,
giving you an overview of what has been done.